doc: update markup for release note "release date" block

This has to be backpatched to all supported releases so release markup
added to HEAD and copied to back branches matches the existing markup.

Reported-by: Peter Eisentraut

Discussion: 2b8a2552-fffa-f7c8-97c5-14db47a87731@2ndquadrant.com

Author: initial patch and sample markup by Peter Eisentraut

Backpatch-through: 9.2

Author: bmomjian

doc/src/sgml/release-10.sgml

@@ -4,11 +4,10 @@
  <sect1 id="release-10">
   <title>Release 10</title>
 
-  <note>
-   <title>Release Date</title>
-   <simpara>2017-09-XX</simpara>
-   <simpara>CURRENT AS OF 2017-04-22</simpara>
-  </note>
+  <formalpara>
+   <title>Release date:</title>
+   <para>2017-09-XX (CURRENT AS OF 2017-04-22)</para>
+  </formalpara>
 
   <sect2>
    <title>Overview</title>